Methods for forming a concave part having a prescribed shape in the surface of a work composed of a metal, synthetic resin, or another material include pressing methods in which a press is used, or chemical etching methods. Pressing methods in which a press is used are generally employed. In these methods, the punch of the press is pressed downwards onto one side of a metal plate having a prescribed thickness, and a recess having a prescribed shape is formed in the metal plate so as to leave a prescribed thickness at the bottom.
In chemical etching methods, a recess is generally formed in a metal plate from one side by well-known chemical etching processes.
However, among the aforedescribed conventional methods, pressing methods in which a press is used involve using a punch to press and form a recess in a metal plate. Metal of a volume equal to that of the recess is therefore pressed to the bottom and towards the periphery, the constituents of the peripheral metal portions are compacted, residual stress is imparted, curling occurs, and flatness is significantly disturbed. A protrusion forms on the other side of the metal plate due to the pressure on the recess, and the flatness deteriorates beyond repair. As a result, the effect of the residual stress is that the recess in particular will warp due to changes over the course of time, and therefore changes in dimensions may occur, causing serious problems from a quality standpoint. A large pressing force is also necessary for pressing out the concave part. A high-power press is necessary and the durability of the punch will worsen. Inevitable problems of increasing costs are therefore presented.
On the other hand, chemical etching methods involve chemically dissolving the metal from one side of a metal plate and forming a concave part. The etching therefore requires large amounts of time and this method is therefore unsuitable for mass production and involves the inevitable problems of cost increases. Etching also does not allow the dissolution of each portion to be controlled separately and problems are therefore presented in that the dimensional accuracy is poor and practical applications are limited.
In order to solve the aforementioned problems with conventional methods, in Japanese Patent No. 3318907 (Patent Document 1) the present applicant proposed and put into practice a method of forming a concave part in a metal plate, in which a concave part can be formed in a metal plate without applying large amounts of stress.
The method of forming a concave part described in Patent Document 1 is characterized in repeating a plurality of times a step for press-forming a concave part from one side of the metal plate, which has a prescribed thickness, in which the concave part has a prescribed shape that is shallower than the thickness of the plate, causing the metal of the convex part to travel to the other side of the metal plate, and forming a convex part having a height that is substantially equal to the depth of the convex part; and a step for cutting the convex part from the proximal end so as to achieve evenness with the surface of the other side. A concave part having a prescribed thickness is gradually formed in one side of the metal plate.    Patent Document 1 (Japanese Patent No. 3318907)